He's still quite fast. If JE is at max speed from mods (+175 supposedly, and no one is), and Sith Assassin is 20ish speed away from max he will still go first. That gap gets smaller at smaller speed increments of course.

Bottom line is that Sith Assassin will have a Sith team go first at equal character panel speeds starting at 200 speed, but he doesn't need that much more speed to go first anyway.